1. Field of the Invention
This invention relates to acicular fine particles of ferromagnetic metal for magnetic recording and a process for producing the same.
2. Description of the Prior Art
In recent years, magnetic powder having a high coercive force and a high spontaneous magnetization has been required for high-performance cassette tapes for audio, compact video tapes, etc., and as a product meeting such requirements, fine particles of ferromagnetic metal have been noted which are obtained by subjecting powder composed mainly of iron oxide or oxyhydroxide (which powder will hereinafter be often referred to as "starting raw material") to heat reduction e.g. in a H.sub.2 stream. In order to control the magnetic characteristics and the stability of oxidation resistance of the iron fine particles, a process has been proposed wherein one or more elements among a group of different elements (mainly metal elements) such as Ni, Co, Al, Si, etc. are adhered onto the starting raw material, followed by heat reduction to prepare fine particles of ferromagnetic metal. The process is concretely a process wherein an aqueous solution of a salt of the above different elements is added to an aqueous suspension of the above starting raw material, followed by changing the pH of the mixture to deposit and adhere the different elements in the form of hydroxide or the like onto the surface of the starting raw material, dehydrating and heat-reducing. In that process, inorganic salts such as chlorides, sulfates, etc. have so far been used as the above salt of the different elements. However, if chlorine ion, sulfuric acid ion or the like present in these inorganic salts remains on the surface of the particles after adhesion, it has a bad effect at the time of heat reduction step and makes the stability of oxidation resistance inferior. Thus, in order to remove them, particles after adhesion have so far been washed with water, but complete removal has been impossible that is, a considerable amount thereof usually remained on the surface of the material. Thus, various characteristics of the resulting fine particles of ferromagnetic metal was limited. The above bed effect which results at the time of heat reduction step refers concretely to sintering and tearing to pieces of the above particles, which will hereinafter be collectively referred to as collapse of particles. Such collapse reduces the uniformity of the particulate form and also produces inferior coercive force (Hc), and squareness (Rs), for powder characteristics, and inferior Hc and Rs for tape characteristics.